1. Field of the Invention
The present invention relates to a superconducting magnet apparatus such as a superconducting magnet for supplying a levitating force to a magnetically levitating train.
2. Description of the Related Art
FIGS. 1 and 2 show this type of conventional superconducting magnet apparatus using an annular superconducting coil. The superconducting coil has, for example, a racetrack-shape. The superconducting coil is manufactured such that a superconductor is formed in a racetrack-shape and epoxy resin is impregnated in the superconductor in a vacuum state.
As is shown in FIGS. 1 and 2, a superconducting coil 1 is housed in a heat-insulating container 2. The container 2 has a storage unit 3 with a racetrack-shape similar to that of the superconducting coil 1. The storage unit 3 includes an inner container 4 containing the superconducting coil 1 and a coolant (typically, liquid helium), an outer container 5 having a normal temperature and containing the inner container 4, and a heat-shield plate 6 provided between the outer container 5 and the inner container 4.
The inner container 4 is normally formed of stainless steel, in order to attain mechanical strength. The inner container 4 is fixed in the outer container 5 via a plurality of heat-insulating supports (not shown). The superconducting coil 1 is firmly fixed in the inner container 4, with a coolant passageway provided. The space between the inner container 4 and the outer container 5 functions as a vacuum heat-insulating layer. The heat shield plate 6 is situated within the vacuum heat-insulating layer, and is normally formed of a metal with high thermal conductivity such as copper or aluminum. The heat shield plate 6 is cooled to an intermediate temperature by means of a cooling system (not shown). In FIG. 1, reinforcing members 7 made of, e.g. stainless steel are provided to reinforce the inner container 4. This structure is adopted to attain higher strength, lighter weight, and smaller size.
However, this superconducting magnet apparatus has the following problem.. While the magnetically levitating train on which this superconducting magnet apparatus is mounted is running, the amount of evaporated coolant in the inner container 4 is greater than in the case where the train does not move. This is due to the following mechanism. When the train starts to run, an electromagnetic force acting between the superconducting magnet apparatus mounted on the train and a levitating/ propelling coil on the ground varies. Owing to the variation in electromagnetic force, the superconducting coil 1 vibrates and consequently a relative displacement occurs between the superconducting coil 1 and the heat shield plate 6. As a result of the relative displacement, eddy current occurs in the heat shield plate 6. Within the varying magnetic field produced by the eddy current, eddy current flows through the wall of the inner container 4. The heat generated by the eddy current in the wall of the inner container increases the amount of evaporated coolant. Thus, in the conventional superconducting magnet apparatus, a large-capacity refrigerator must be mounted on the train.
In order to overcome the above problem, it is thought that the inner container is formed of a non-magnetic, non-electrically-conductive material such as fiber-reinforced plastic. In this case, however, in order to attain a mechanical strength equal to that of the stainless steel inner container, the size of the inner container must be increased.
As has been described above, when the conventional superconducting magnet apparatus is mounted on, in particular, a magnetically levitating train, the amount of evaporated coolant increases and the large-capacity refrigerator must be required.